1. Field of the Invention
This invention relates to an optical disc, such as a digital audio disc (so-called compact disc) or an optical video disc. More particularly, it relates to an improved optical disc having an the reflective film.
2. Description of the Prior Art
The optical disc, such as the digital audio disc or the optical video disc, is repaired by forming a refractive film on the transparent substrate, generally formed of polycarbonate or acrylic resin, and forming a hard protective film on the so-formed Al reflective film.
FIG. 1 shows the cross-section of this optical disc. The optical disc 1, shown in FIG. 1, is constituted by a transparent substrate 2, formed of a synthetic resin, such as polycarbonate or acrylic resin, by reason mainly of mass-producibility, as mentioned above, a reflective film 3 of aluminum or the like, formed on the substrate 2 by sputtering or evaporation, and a protective film 4 formed on the reflective film 3 for protecting the reflective film by spin coating a UV curable resin. In the case of a reproduce-only optical disc, such as a digital audio disc or an optical video disc, a pattern of projections and recesses, corresponding to the recorded signals, are formed on one surface of the transparent substrate 1, more correctly, on the surface thereof opposite to the signal reading surface, that is, on the surface thereof on which the reflective film 3 is formed. In the case of a write-once type optical disc on which information signals can be recorded, or an erasable optical disc, grooves are formed on the same one surface of the transparent substrate 1 as that of the reproduce-only optical disc. Although An may be used as the material for the reflective film 3, instead of Al, the latter is usually preferred because it is less expensive.
Besides being inexpensive, the Al film is superior in reflectivity and hence used extensively.
Meanwhile, the Al reflective film has a deficiency that the film of Al, being metal, is subject to changes with lapse of time on prolonged storage due to extraneous factors, and therefore leaves much to be desired.
For this reason, attempts have been made to form an oxide film on the surface of the Al reflective film by previous oxidation to provide an aluminum-aluminum oxide-aluminum three-layer structure to suppress deterioration with lapse of time.
However, this three-layer structure of the Al reflective film is also not fully satisfactory in durability because, if there be any flaw in the Al reflective film, corrosion tends to be started thereat to proceed into the metal aluminum of the mid layer of the three-layer structure.